A brief history of sound mirrors

Sound or acoustic mirrors were one of the first early warning detection systems invented to give advanced notice of an approaching enemy aircraft. These worked by focusing the sound from the plane’s engine so it could be heard before it was visible.

Powered flight

Louis Bleriot was the first person to fly across the channel in 1909; this leap forward in aviation technology meant that Britain’s status as an island nation was not enough to prevent attack from Europe. The 22 mile crossing to France no longer offered protection from attack and new ways of protecting ourselves were required.

The early story of sound mirrors is not clear, but it is believed that the first experiments were conducted in 1915. The first mirrors or ‘dishes’ were cut directly into the chalk to try and reflect the sound. These early beginnings soon gave way to larger and more complicated structures made of concrete.

How sound mirrors work

Sound mirrors worked using a curved surface to concentrate sound waves into a central point, which were picked up by a sound collector and later by microphones. An operator using a stethoscope would be stationed near the sound mirror, and would need specialist training in identifying different sounds. Distinguishing the complexity of sound was so difficult that the operators could only listen for around 40 minutes.

Fan Bay Sound Mirrors

There are two mirrors at Fan Bay; the first constructed in 1917 is believed to be one of the oldest surviving sound mirrors in the country; the second is larger and was built at some point in the 1920s. Both mirrors were initially cut into the chalk and lined with concrete to form the distinctive dish shape.

A quick break from digging
Volunteer diggers sat by a half buried sound mirror

Loss and re-discovery

The sound mirrors were covered up in the 1970s as part of a programme to clear eyesores on the landscape; 600 cubic meters of soil were dumped over the site to completely obscure the mirrors from view. We are now working to undo that damage by removing the soil and debris to once again reveal the mirrors.